As the semiconductor industry introduces new generations of integrated circuits (IC) having higher performance and more functionality, the density of the elements forming the ICs increases, while the dimensions, sizes and spacing between components or elements are reduced. In the past, such reductions were limited only by the ability to define the structures photo-lithographically, device geometries having smaller dimensions created new limiting factors. For example, for any two adjacent conductive features, as the distance between the conductive features decreases, the resulting capacitance (a function of the dielectric constant (k value) of the insulating material divided by the distance between the conductive features) increases. This increased capacitance results in increased capacitive coupling between the conductors, increased power consumption, and an increase in the resistive-capacitive (RC) time constant. Therefore, the continual improvement in semiconductor IC performance and functionality is dependent upon developing materials with low k values.
Since the material with lowest dielectric constant is air (k=1.0), low-k dielectric materials typically comprise porous materials. Furthermore, air-gaps are formed to further reduce effective k value of interconnect structures.
FIGS. 1A through 1C illustrate a conventional process for forming an interconnect structure with air-gaps. Referring to FIG. 1A, copper lines 4 and corresponding diffusion barrier layers (not shown) are formed in an inter-metal dielectric 6, which has a low k value, and contains a high concentration of carbon. During the formation of copper lines 4, portions 8 of inter-metal dielectric 6, which are exposed during the formation of copper lines 4, are damaged, and hence have a low concentration of carbon. The damaged portions 8 may be etched by HF to form air-gaps 10, as illustrated in FIG. 1B. Subsequently, as shown in FIG. 1C, dielectric layer 11 is formed, hence sealing air-gaps 10.
Although the formation of air-gaps 10 reduces the parasitic capacitance of the interconnect structure, the conventional process suffers drawbacks. Due to the formation of air-gaps 10, no dielectric layer is formed against sidewalls of copper lines 4. Without the back-pressure provided by the dielectric layer, electro-migration (EM) increases, and time dependent dielectric breakdown (TDDB) performance of the interconnect structure is adversely affected. A further problem is that in subsequent processes for forming overlying vias on the copper lines 4, if a misalignment occurs, the vias may land on air-gaps 10, resulting in discontinuity of the diffusion barrier layer. This causes copper to be in direct contact with low-k inter-metal dielectric layer 6, hence the diffusion of copper into low-k inter-metal dielectric layer 6.
Accordingly, what is needed in the art is an interconnect structure that may incorporate air-gaps thereof to take advantage of the benefits associated with reduced parasitic capacitances while at the same time overcoming the deficiencies of the prior art.